Replacing steel components with fiber-reinforced plastic components, or composite components, is an important tool for cost- and weight-optimized solutions for future motor vehicles or automobiles. One example of this is leaf springs made of fiber-reinforced composite materials, which weigh up to 60% less than comparable steel components.
In addition, the mechanical properties of fiber-reinforced composite materials offer further advantages over conventional materials. Leaf springs made of fiber-reinforced composite material, also known as composite leaf springs, absorb energy more easily than steel and, when installed in the spring systems of the shock damping arrangement, improve the ride comfort of the occupants of the vehicle. In addition to greater fatigue resistance than steel springs, the onset of material failure in composite materials would be gradual and would therefore be identifiable in advance, making it possible to avoid the sudden, catastrophic failure of metal parts. Furthermore, fiber-reinforced plastic leaf springs are easily capable of withstanding typical ambient conditions. They are corrosion-resistant and resistant to salt damage in winter as well as to oil, gasoline and battery acid. In contrast to metal, it is also not necessary for composite leaf springs to be coated with a protective anticorrosion paint.
Composite leaf springs are usually produced by arranging reinforcing fibers in a plastic material such as an epoxy matrix or a matrix of other polymer resins. Typically, the fiber materials are provided in webs or rolls and are cut to size in a die prior to lay-up in the molds. They are then laid in the mold at a distance from one another, the mold being then filled with resin or another comparable polymer. A chemical reaction, by which the leaf spring is formed, is initiated by heat treatment. Another typical composite material is prepreg. In the case of prepreg, an alternative production method is used. In that context, a fiber-matrix semifinished product made of a textile pre-impregnated with reactive resins, or a fiber-reinforced resin system is formed to give a composite leaf spring by pressing and heat-treating.
Moreover, for the installation or assembly of the leaf springs, one or more so-called eye bushings are provided at the ends of the leaf springs. The integration of these cylindrical bushings at the ends of the leaf springs made of fiber-reinforced plastic material, without disrupting the fiber profile in order to obtain a robust, long-lived and cost-effective plastic leaf spring with integrated attachment devices, is a challenge.
In the prior art, US 2012/0211931 A1 discloses a composite leaf spring made of a fiber-reinforced body that is embedded in a cured resin, having a central section and opposite end sections. The fibers are arranged in multiple layers of a web which extend essentially completely, in the form of a continuous web, over the central section and any end sections of the composite leaf spring. The web material itself has multiple strands that extend in the longitudinal direction along the web, i.e. in the direction of the length of the fiber-reinforced body. Furthermore, the web comprises a row of elements running transversely, which extend transversely (obliquely or generally perpendicular) relative to the length of the fiber-reinforced body.
Further, the teaching of U.S. Pat. No. 3,900,357 describes various spring configurations and production methods for leaf springs made of fiber-reinforced composite materials and the possibilities thereof for application in motor vehicles. What is disclosed is the use of fine filaments or fibers as stress absorbers. The fibers are laid flat against one another and are held together by means of a suitable matrix material, wherein in a single ply these fibers extend essentially in the same direction. Multiple plies, pre-impregnated with a matrix material, can be combined to produce a multi-ply laminar structure. Then, the leaf spring can be encapsulated in the manner of a helix with two layers of composite material. In that context, inter alia initially the first spiral-shaped layer is wrapped in a left-handed helix, and then the second spiral-shaped layer is wrapped in a right-handed helix around the entire leaf spring.
U.S. Pat. No. 4,414,049 has the object of specifying a method for producing a leaf spring with a variable cross section and a variable thickness. In that context, the filament material is braided as a single coherent strand between bolts or bushing elements so as to form a leaf spring. In that context, the more bolts or bushing elements are incorporated, the larger the cross section of the leaf spring.
U.S. Pat. No. 9,194,451 B2 relates to a method for producing a leaf spring in a fiber composite material, in which a bearing eye is formed at at least one axial end. In that context, first a strip of prepreg is prepared and a bearing eye is configured at the respective ends of the strip. In order to form the basic structure of the leaf spring, a plurality of prepreg strips are arranged one on top of another to form a prepreg stack. A leaf spring produced in that manner consists of just a single material and has, at at least one of the axial ends, a bearing eye that is integrated into the leaf spring. Since the leaf spring contains no pairings of different materials, in operation the static and dynamic forces that are transmitted to the leaf spring via the at least one bearing eye are easily taken up in the central portion of the leaf spring as tensile and compressive stresses and thus ultimately converted into heat energy.
JP 57124141 A increases the strength of an end-position eyelet part of a leaf spring, in that both end parts of the leaf spring are wrapped around the beginning of a bushing and the remaining portions are stitched or woven together with the central portion of the leaf spring. This makes it possible to achieve great strength in the region of the eyelet parts, even with simple manufacture.
U.S. Pat. No. 4,565,356 illustrates a possibility for coupling the fiber material with the bushings in order to reduce stress concentrations in the leaf spring. In that context, the bushing comprises a tubular section with a central opening, wherein rib bundles extend outward from the tubular section.
U.S. Pat. No. 4,749,534 also discloses a production method for a composite leaf spring, which aims to better distribute the load by varying the breadth of the leaf spring. The leaf spring produced in that context is also equipped with an attachment eye. The coupling between the attachment eye and the leaf spring is established by means of metal inserts.
EP 0 215 365 A2 relates to a fiber-reinforced plastic leaf spring, in which cylindrical bushings are present at the ends and are surrounded by fiber strands, and a retaining device is attached in the middle. This invention provides that the fiber strands extending from bushing to bushing are formed so as to surround the bushings and, after passing around the bushings, end at the continuous fiber strands.
A method known from U.S. Pat. No. 4,468,014 provides a composite leaf spring which is produced by positioning layers spaced apart from one another in a mold and subsequently pouring material into the space or injecting the core material into the space between the layers. Preferably, shells which form the mold have a constant breadth and thickness in order to reduce production costs. In most common automotive applications, the spring would have a point of maximum thickness in the middle section, where the bending moment is greatest, and would taper toward each end.
U.S. Pat. No. 4,696,459 provides a plastic leaf spring with two plate-shaped reinforcing elements. According to one advantageous embodiment, the reinforcing elements are connected to one another and to the end of the leaf spring by bolts.
In light of the prior art presented here, production methods for leaf springs made of fiber-reinforced plastic or composite material still leave room for improvement. The invention is therefore based on the object of providing an improved production method for high-strength, long-lived, lightweight and cost-effective leaf springs made of fiber-reinforced plastic with integrated eye bushings, wherein in particular identical or uniform wall thicknesses are to be achieved everywhere in the leaf spring. The invention is also based on the object of providing such a leaf spring.